Many spray painting machines have been introduced to paint mass-produced items such as automotive bodies. Such paint spraying machines have typically been limited in their use in that they must be backed up by human operators, who must touch up areas missed by the machines. Such machines also tend to be wasteful of paint and are only designed to paint with horizontal and/or vertical paths on a reciprocator system. Coating an object fully, especially if it has complex contours, requires movements in depth as well as lateral movements. The use of a wrist is significant in that it is often necessary for an operator to angle a paint spray gun in a particular manner to deposit paint on a heavily contoured surface.
Many of the prior art spray painting machines include a battery of spray guns fed from large capacity, centralized paint reservoirs which manage to paint a large percentage of the exterior surface to be covered. Less accessible areas, such as wheel arches, the interiors of the trunk or engine compartment and door edges had to be painted by operators who looked for unpainted areas as the car body left the automatic painter.
Numerous prior art patents disclose painting machines including electrical painting apparatus such as the U.S. Patent to Chapman U.S. Pat. No. 2,858,947; the U.S. Patent to Shelley et al U.S. Pat. No. 3,007,097; the U.S. Patent to Pierson et al U.S. Pat. No. 3,481,499; the U.S. Patent to Richter U.S. Pat. No. 4,030,617; the U.S. Patent to Yoshio U.S. Pat. No. 4,113,115; the U.S. Patent to Burns et al U.S. Pat. No. 4,196,049; the U.S. Patents to Shum U.S. Pat. Nos. 4,398,863 and 4,407,625; the U.S. Patent to Jacot-Descombes et al U.S. Pat. No. 4,424,472 and the U.S. Patent to Gorman U.S. Pat. No. 4,424,473. The U.S. Patents to Pollard U.S. Pat. Nos. 2,213,108 and 2,286,571 both disclose electrical robots for paint spraying. The U.S. Patent to Stricker U.S. Pat. No. 4,170,751 likewise discloses an electric apparatus associated with paint spraying robots.
There are many reasons for using spraying robots to provide high-quality painted finishes on a mass-produced item. Robots are desirable in that they are able to cope with the hostile painting environment; they allow the painting process to proceed with less total energy being expended; and they improve paint quality which, in turn, eventually results in reduced material and labor costs. The above advantages are particularly important in painting car bodies where production rates are expected to be high, and there is a relatively small amount of time available for the paint to be applied and completed in not one, but several coats.
A spraying robot can also be used in the enameling and/or powdering of surfaces. For example, application of vitreous enamel to plastic bathtubs utilizes many of the same spray techniques as used in the application of paint to an automotive body.
Robots which have been used for spray painting comprise continuous path machines which emulate the action of a human operator. Such robots are typically taught by having an expert painter lead each robot in its learning mode through an actual paint job.
Paint robots have been used for some time to apply paint to large parts such as car bodies. Robots, in general, have a small work envelope or area within which they can move. Therefore, to increase the work envelope large enough to paint these large parts, the robot has to be mounted on either a rail, table or slide by means of a carriage. Of the three methods of obtaining the larger work envelope, the rail is preferred because a rail occupies much less space than tables or slides and the rail does not create and/or distribute dust as much as tables or slides. An example of such a system is shown in the U.S. Patent to Bartlett et al U.S. Pat. No. 4,312,535.
The extra increase in work envelope provided by one of these three devices allows the robot to paint one side of an entire car with the car either stationary or moving. If the car is moving, the extra axis tracks the car as it moves through the booth. By adding an additional rail, table or slide on the opposite side of the booth, both sides of the car can be painted.
The average spray booth in the automotive industry has been standardized to have a relatively small width. This small width typically restricts the size and movements of any robot which is to be used in such a spray booth.
The area within any paint spray booth must be free from any shelves, pockets, loose hoses or standard street clothing because they collect and/or distribute dust or lint. Any dust or lint in the area will find its way onto the car or large object being painted which, in turn, ruins the paint job.
The top or front of a typical rail is usually covered with a "way cover" which connects the moving carriage to the ends of the rails. This cover is required to protect the bearings, ways, and drive mechanism from ths hostile paint environment. The way cover comes in a variety of types, including telescopic tubes or plates, accordian and window shade. Way covers can be made of either an elastomer such as rubber or a sheet of stainless steel.
Way covers have traditionally been a high maintenance item for paint shops. The paint tends to accumulate on them causing the adjacent layers of the material to stick together, eventually causing fatigue in the cover. As the way cover moves and scrapes itself, the paint and dust may fall on the freshly painted car body resulting in defects. The defects require sanding, cleaning, and repainting.
Most paint booths are of a down-draft type which means that the air blows through filters in the top of a paint booth, around the car body and out the bottom of the paint booth. The rail is an obstruction to the air flow and causes an increase in air speed which, in turn, causes a partial vacuum in front of the rail. The partial vacuum may cause loose paint particles from the way cover to be deposited on the car body. The rail also causes turbulent air flow which upsets the streamlines in the booth and increases the chance of getting dirt on the car body. The volume of circulated air is rather large and requires a substantial amount of energy to circulate, heat, cool and dehumidify it.